Does installing SharePoint 2010 on virtual machines, for Production use with around 20,000+ users, meet best practice recommendations?

My question, as the title indicates, is "Does installing SharePoint 2010 on virtual machines, for Production use with around 20,000+ users, meet best practice recommendations?"

Sharepoint 2010 is being used to develop the website for our state university, with a student population of about 20,000 users.

We had our SharePoint environments designed, installed, and configured by an outside consulting company, which we trusted to follow SharePoint 2010 best practices.

Now we're experiencing performance issues in various pages and in the ADFS sign-in process.

We started researching what are the best practices for SharePoint 2010, and the only examples of using Virtual Machines for a SharePoint environment are for use in learning SharePoint, and for individual developer environments.

We have not been able to find any recommendations for or against using Virtual Machines in a production environment with a large number of users.

Any advice, feedback, constructive criticism is appreciated.

Thank you.